Microwave radiation has been found to have a number of uses, particularly in the areas of communications and as a means of transmitting energy to a product to facilitate processing of that product. The latter application generally involves a unit that provides a processing cavity for receiving the product. Wave guides then direct microwaves from a microwave transmitting source into the processing cavity. In the processing cavity, the microwaves will either be absorbed or reflected, depending upon the various surfaces and materials they come in contact with. Ideally, the product will uniformly absorb all of the radiation. To date, however, this goal has not been met.
The energy field established in the processing cavity by the microwave transmitting source does not have a completely homogenous density. This often results in hot and cold spots and contributes to uneven processing of the product. A number of prior art devices purport to reduce such nonuniformity. In one, the microwave transmitting antenna may be rotated at the exit of the wave guide into the processing cavity. The notion here is to disperse the microwave energy field at different angles. This approach has not fared well, in part because the mechanism operates on an assumption that a beam theory of microwave behavior will adequately describe microwave behavior in a closed cavity. In fact, such a presumption falls short of clearly describing microwave behavior in a close-quartered environment.
Another prior art device known as a mode stirrer provides a grounded metal surface in the processing cavity area. By rotating the mode stirrer, the processing cavity may be tuned to a fairly uniform energy field. To the extent that such devices rotate automatically, however, the mode stirrer may actually detract as much or more as it contributes to uniformity.
Yet another prior art solution to achieve uniformity has been to move the product within the processing cavity during processing. By such movement, the nonuniformities in the energy field would hopefully be averaged out.
Such product movement has generally been accomplished in one of two ways. The product may either be moved up and down, or it may be rotated as on a turntable. The up and down movement may not be successful in improving uniformity, as the energy field may follow such motion and defeat the purpose of moving the product. Rotating the product, however, has been successful. The energy field has somewhat more difficulty in matching rotational movement, and therefore a better averaging of nonuniformity exposure occurs. Even with rotational movement, however, experience has shown that layers of uneven processing can be found in processed products.
Another problem related to uniformity in the area of microwave processing units involves microwave reflections. In particular, if microwave reflections from the processing cavity make their way back to the transmitting source with sufficient energy, the transmitting source will likely be damaged. To protect against this, three port circulators or isolators have been placed in the wave guides to block such reflections. Although expensive, these devices work quite well and do protect the transmitting source from stray reflections.
Reflections may also be reduced by off-loading the processing cavity, to thereby absorb within the processing cavity certain quantities of energy. Unfortunately, off-loading inherently reduces efficiency as it removes energy that might otherwise be usable for processing. The prior art lacks, however, a means of reducing possibly dangerous reflections and at the same time increasing efficiency of the unit.